Oscillator circuit for high-frequency dielectric heating



May 23, 1950 L. D. DRUGMAND OSCILLATOR CIRCUIT FOR HIGH-FREQUENCY mELEcTRic: I-IEA'HNG mm Nov. -10, 1945 A N I a Mama/u Patented May 23,1950

OSCILLATOR CIRCUIT FOR HIGH-FRE- QUENCY DIELECTRIC HEATING Lester D. Drugmand, Milwaukee, Wis., assignor to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application November 10, 1945, Serial No. 627,871

6 Claims.

This invention relates to an improved oscillator circuit for high frequency dielectric heat-- mg.

A primary object of the invention is to provide'an improved and simplified high-frequency heating system for dielectric articles.

Another object is to provide a circuit for high frequency dielectric heating wherein the necessity for the usual expensive blocking condensers is avoided.

A more specific object is to provide such a circuit wherein the inherent capacity between the plate and grid of each tube performs the function ofa blocking condenser.

Another object is to provide such a circuit which inherently acts to substantially prevent subjection of the wor plates or heating electrodes to the low frequency current from the source of power supply, while insuring a relatively low impedance path for the flow of high frequency current to such electrodes.

Other objects and advantages of the invention will hereinafter appear.

The accompanying drawing illustrates diagrammatically a preferred embodiment of the invention; it being understood that the embodiment illustrated is susceptible of modification in respect ofcertain details thereof without departing from the scope of the appended claims.

In the usual form of tuned plate, tuned grid oscillator circuit employed for high frequency dielectric heating the plate tank circuit was utilized to provide the output of high frequency current; such :an arrangement involving the necessity for use of a pair of expensive blocking condensers connected in series with the respective plates and in proper circuit relationship to a pair of high frequency choke coils.

' I have discovered that the use of such condensers in the oscillator circuit may be eliminated by merely utilizing the grid tank circuit for the output of the high frequency current employed for high frequency dielectric inductive heating.

Referring to the drawing, the numerals 5 and 6 respectively-designate the positive and negative terminals of a suitable source of supply (not shown) of high-voltage current, which preferably is direct current; but low frequency alternating current may be employed if desired. The negative terminal 6 of said source is shown as having a ground connection at I. The oscillator circuit includes a pair of tubes 8 and 9, preferably 'of the high-vacuum triode type. The anodes 8 and 9- are respectively connected with condenser I2 is connected in parallel with winding l0.

The cathodes 8 and 9 of tubes 8 and 9 have a common connection l3, through a suitable ammeter I4, with the aforementioned negative terminal 6. v

The grids 8 and 9 of tubes 8 and 9 are respectively connected by conductors l5 and 16 with taps l'l and l'l of an inductance I1, said taps being preferably spaced like distances from the center tap ll? of said inductance. Said center tap I! is connected by conductor l8, through a condenser I9 and resistance 20 which are connected in parallel with each other, to ground at 2|. The opposite ends of inductance I! are respectively connected with the upper and lower electrodes 22 and 23, whereby the latter are supplied with high frequency current for heating a dielectric article 24 positioned therebetween, by virtue of its dielectric loss. When the dielectric article 24 (such as thermoplastic and thermosetting insulation) is preformed to the cylindrical shape illustrated diagrammatically in the drawing, the lower stationary electrode 23, and also preferably the movable upper electrode 22, will be of curved or trough shape in transverse cross section, as indicated. Metal electrodes of such form are disclosed and claimed in my prior application, Serial No. 571,778, filed January 8, 1945, now Patent Number 2,472,370, and form no part of the present invention. Thus it will be apparent to those skilled in the art that electrodes of any suitable or desired shape may be employed herein, according to the shape of the dielectric articles to be heated and/or the character of the insulating material of which the same are composed.

As aforeindicated the lower electrode 23 is stationary, whereas the upper electrode 22 is movable, either manually or automatically, toward or away from the dielectric article sup-ported by the lower electrode to vary the value of the high frequency current induced in the article to effect heating thereof. As is well known the electrodes 22 and 23 act in the manner of condenser plates with reference to each other and with reference to the body or bodies of dielectric material interposed therebetween.

The operation of the system is as follows: with terminals 5 and 6 connected to a source of supply of current of a suitable relatively high voltage the input to the circuit herein disclosed may be controlled by manual adjustment or preadjustment of the variable condenser l2. Such adjustment will, of course, depend upon the maximum rate of heating desired for the dielectric articles 24 (such as insulation preforms) interposed between electrodes 22 and 23; and such rate will in turn be selected largely in accordance with the particular character or composition of the insulating material and the shape of the preforms.

The adjustment of condenser l2 thus serves to tune the circuit of each anode .or plate 8 9, whereas the circuit of each grid 8, 9 is tuned by the relative positions of the electrodes 22, 23 with respect to each other and with respect to the dielectric material interposed therebetween. The choke coil H functions in a well known manner to prevent transmission of the high frequency current generated in the system to the aforementioned source connected to terminals and 6. The grids 8 and 9 function in the usual manner to control the rate of flow of high frequency alternating currentbetween the anodes 8 9 and their respective cathodes 8 9 On the other hand, the inherent capacities between the grids 8, 9, and their associated anodes 8 9, function in the manner of blocking condensers, thus preventing any appreciable transmission of the relatively low frequency current from the source of power to the electrodes 22 and 23. Such capacitive relationship of the anodes 8 9 to their respective grids 8, 9 nevertheless insures maintenance of relatively low impedance paths for the flow of high frequency current to the electrodes 22 and 23.

As will be apparent to those skilled in the art, the oscillator circuit aforedescribed is extremely simple, and by providing for elimination or omission ofthe aforementioned blocking condensers ordinarily employed in such circuits both the initial and maintenance cost thereof is greatly reduced.

The ammeter l4 serves as an indicator of the value of high frequency current: supplied to the dielectric material 24 from electrodes 22 and 23, and dissipated therein in the form of heat; and hence by observing the instantaneous reading of said ammeter the operator may manually raise or lower the upper electrode 22 until the desired value of output of high frequency current has been attained. I have diagrammatically shown at 25 an excess length of conductor to compensate for movement of electrode 22; it being understood that suitable means (not shown) may be provided to prevent accidental movement of said electrode from any adjusted position thereof.

I claim:

I.- In an oscillator circuit of the tuned plate, tuned grid type, said circuit'including apair-of vacuum tubes of the triode type, an inductance to opposite ends of which the anodes of the tubes are respectively connected, said inductance having a center tap connection with one terminal of a suitable source of power supply, the cathodes of the respective tubes being connected'to the other terminalof said source and to ground, the combination with a secondinductance having, a center tap connection to ground, of means for connecting the grids of the respective tubes to symmetrically arranged taps onsaid lastmentioned inductance, and a pair of spaced electrodes connected to opposite end of said last mentioned inductance and adapted to provide for high ire- 4 quency heating of dielectric material interposed therebetween.

2. In an oscillator circuit of the tuned plate, tuned grid type, adapted for high frequency heating of dielectric materials, said circuit including a pair of tubes of the high vacuum triode type, an inductance, an anode tank circuit connected through said inductance to a source of high voltage current, the combination with a second inductance, of a grid tank circuit connected to symmetrically spaced taps on said second inductance, and a pair of spaced electrodes electrically connected to the respective terminals of said last mentioned inductance to provide for high frequency heating of dielectric material interposed between said electrodes, the inherent capacity between each anode and its associated grid providing the effect of a blocking condenser whereby the flow of relatively low frequency current from the source of power to said electrodes is substantially prevented while affording relatively low impedance paths for the high frequency current to be supplied to said electrodes.

3. In an oscillator circuit of the tuned plate, tuned grid type comprising a, pair of tubes of the high vacuum triode type and an anode tank circuit connected through an inductance to a source of high voltage current, in combination, a grid tank circuit including a second inductance having a ground connected center tap, a load comprising a pair of high frequency electrodes electrically connected to the respective terminals of the last mentioned inductance, and means affording electrical connections between the grids of the respective tubes and taps spaced symmetrically with respect to the center tap of said second inductance and the respective terminals of the latter, the inherent capacity between each anode and its associated grid providing the effect of a blocking condenser whereby the flow of relatively low frequency current from the source of power to said high frequency electrodes is substantially prevented, while affording relatively low impedance paths for the high frequency current to be supplied to said high frequency electrodes.

4. In an oscillator circuit of the tuned plate, tuned grid type including a pair of high vacuum tubes of the triode type, wherein the anodes of the tubes are respectively connected to opposite ends of an inductance having a center tap connection with one terminal of a suitable source of power supply and wherein the cathodes of the tubes are respectively connected jointly to the other terminal of said source and to ground, the combination with a second inductance having a center tap connection to ground, a load comprising a pair of high frequency electrodes connected to opposite ends of said last mentioned inductance, and means for connecting the grids of the tubes respectively to symmetrically arranged taps between the center tap and theopposite ends of said last mentioned inductance, said circuit being characterized by the fact that the inherent capacities between the anodes and their associated grids serve as blocking condensers with respect to the relatively low frequency current from the source of power while acting as parts of relatively low impedance paths for the high frequency current supplied to said high frequency electrodes.

5. In anoscillator circuit of the tuned plate, tuned grid type comprising a pair of tubes of the high vacuum triode type and an anode tank circuit connected through an inductance to a source ofhigh voltage current, in combination, a grid tank circuit including a second inductance having a ground connected center tap, a load connected to the respective terminals of the last mentioned inductance, and means affording electrical connections between the grids of the respective tubes and taps spaced symmetrically with respect to the center tap of said second inductance and the respective terminals of the latter, the inherent capacity between each anode and its associated grid providing the effect of a blocking condenser whereby the flow or" relatively low frequency current from the source of power to said load is substantially prevented, while affording relatively low impedance paths for the high frequency current to be supplied to said load.

6. In an oscillator circuit of the tuned plate, tuned grid type including a pair of high vacuum tubes of the triode type, wherein the anodes of the tubes are respectively connected to opposite ends of an inductance having a center tap connection with one terminal of a suitable source of grids of the tubes respectively to symmetrically arranged taps between the center tap and the opposite ends of said last mentioned inductance, said circuit being characterized by the fact that the inherent capacities between the anodes and their associated grids serve as blocking condensers with respect to the relatively low frequency curent from the source of power While acting as parts of relatively low impedance paths for the high frequency current supplied to said load.

LESTER D. DRUGMAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,068,799 Guyer Jan. 26, 1937 2,149,387 Brown Mar. 7, 1939 2,179,261 Keller Nov. 7, 1939 2,239,069 Worden et a1. Apr. 22, 1941 2,248,840 Wilkoff July 8, 1941 2,318,061 Dailey May 4, 1943 2,406,839 Labin et a1 Sept. 3, 1946 2,438,595 Zottu Mar. 30, 1948 

